Musellifer leasiae, Sørensen & Araújo & Macheriotou & Braeckman & Smith & Ingels, 2025

Sørensen, Martin V., Araújo, Thiago Q., Macheriotou, Lara, Braeckman, Ulrike, Smith, Craig R. & Ingels, Jeroen, 2025, A new gastrotrich, Musellifer leasiae sp. nov. (Paucitubulatina: Muselliferidae), from Antarctica-the first Muselliferidae species description from the Southern Hemisphere, Zootaxa 5647 (4), pp. 359-370 : 362-367

Musellifer leasiae sp. nov.

urn:lsid:zoobank.org:act:818C0779-F781-493F-B287-F0FD5A1F81F0

( Figs 2–5 ; Table 1)

Material. Holotype, adult, collected from mud on April 11, 2016, at CRS St. 1793 ( Fig. 1 A–C ), from 701 m depth in the Gerlache Strait, Antarctic Peninsula, Antarctica (position: 64°39.53′ S, 62°55.03′ W), mounted in Fluoromount G, deposited at the Natural History Museum of Denmark, under catalogue number NHMD-1801023. Paratypic material includes 1 adult, same data as holotype, deposited under catalogue number NHMD-1801024; 2 adults, collected from mud on December 6, 2015, at CRS St. 1716, from 551 m depth in IBB, Andvord Bay , Antarctic Peninsula, Antarctica (position: 64°52.36′ S, 62°25.49′ W), mounted in Fluoromount G, deposited under catalogue numbers NHMD-1801025 to 1801026; 1 adult, collected from mud on April 10, 2016, at CRS St. 1790, from 532 m depth in MBA, Andvord Bay , Antarctic Peninsula, Antarctica (position: 64° 51.49′ S, 62° 34.01′ W), mounted in Fluoromount G, deposited under catalogue number NHMD-1801027; 1 adult, collected from mud on April 15, 2016, at CRS St. 1809, from 694 m depth in the Gerlache Strait, Antarctic Peninsula, Antarctica (position: 64°39.59′ S, 62°55.09′ W), mounted in Fluoromount G, deposited under catalogue number NHMD-1801028; 9 adults, collected from mud on April 7, 2016, at CRS St. 1776, from 551 m depth in IBB, Andvord Bay, Antarctic Peninsula, Antarctica (position: 64°52.53′ S, 62°33.90′ W), mounted for SEM, deposited under catalogue numbers NHMD-1801029 to 1801037 . Additional material includes 2 adults, collected from mud on April 6, 2016, at CRS St. 1773, from 553 m depth in IBB, Andvord Bay , Antarctic Peninsula, Antarctica (position: 64°52.35′ S, 62°25.88′ W), mounted for SEM, and stored in the first authors personal reference collection .

Diagnosis. Musellifer with total body length of 322 to 415 µm, and body divided into head (U0–U17), neck (U17–U20), trunk (U20–U70) regions, and terminal furca, gradually tapering into adhesive tubes (U70–U100). Head with well-defined muzzle. Scales differentiated into morphologically different head-, trunk- and furca scales. Head scales are three-lobed with a median keel, extending into a spiny distal process. Trunk scales arranged imbricate, with anterior ones forming weakly rhomboidal shield-shaped scales with pair of short spiny processes flanking a long, posteriorly directed spine; more posterior trunk scales are balloon shaped, with indistinct paired processes, but a well-developed posteriorly directed spine. Imbricate furcal scales narrow, cone-shaped to rectangular, with short, paired spiny processes flanking a median spine.

Etymology. The species is dedicated to Dr Francesca Leasi in recognition of her numerous contributions to gastrotrich taxonomy and morphology.

Description. Holotype 415 µm in total length; paratypes 322 µm to 400 µm in total length; average length of all types: 368 µm.

External morphology

Holotype: Body slender, divided into pointed head (U0–U17), weakly marked neck (U17–U20), nearly parallelsided trunk (U20–U70), and gradually tapering furcal branches (U70–U100) ( Figs 2A, B , 3A , 4A ). Head with pronounced muzzle, with a cuticular mouth ring surrounding the terminal mouth ( Fig. 4B, E ). Behind the muzzle, the head gradually widens, reaching its maximum width, 53 µm, at U11. Cilia are distributed around the terminal part of the head, around the buccal cup, and extend ventrally, forming an elongate, ciliary field at the ventral surface to U14 ( Figs 2B , 3B, C , 4B–D ) and dorsally to U06. The neck region is indistinct, measuring 50 µm at its narrowest point (U19). The anterior part of the trunk is only slightly broader, 54 µm, and it widens gradually towards its widest point, 68 µm, at U49. Imbricate scales are arranged uniformly along the body axis ( Fig. 3D ), but are interrupted by two ventrolateral ciliated furrows (a paired column of locomotory cilia) that extend longitudinally from a position near the posterior margin of the ventral ciliary field (U15) to a point about halfway down the trunk (U48) ( Figs 2B , 3C, E , 5A, B ). The furca (inclusive adhesive tubes) is 130 µm long, and each furcal branch is 23 µm broad at its proximal base. From this point (U70) they taper gradually until U84, where they have narrowed to 3 µm width, and articulate with adhesive tubes of the same width ( Figs 2A, B, F–I ). The adhesive tubes maintain their width of 3 µm to their distal tips. A clear distinction between furcal bases and adhesive tubes is not possible, since the entire tubes are also covered scales ( Figs. 4A , 5J–L )

Scales (holotype and paratypes) are arranged in approximately 26 columns (8 dorsal, 2x5 lateral, and 8 ventral including those between the two columns of locomotory cilia), with about 45 scales per column. The morphology of the head- and trunk scales differ considerably, and at least four different scale shapes are observed ( Fig. 2C ).

Head covered of three-lobed keeled scales dorsally and ventrolaterally from U09–U16. The two posterior lobes have pointed tips, 1.3 µm long. The thick keel rises close to the rounded anterior lobe, and the highest point of the keel measures 3 to 4 µm. A long spiny process at the distal portion of the keel extends to 4 to 6 µm in total length.

Ventral, lateral and dorsal surfaces of the trunk follow the same scale arrangement. The lateral sides of the head from U10, and the anterior part of the trunk, from U15 to U55 are covered by smooth and weakly rhomboidal shield-shaped scales (i.e. stingray-shaped sensu Trokhymchuk & Kieneke 2024) with a broadly rounded anterior margin, a pair of lateral small “notches” in the middle, and a pair of short spiny processes (1 µm in length) at the posterior end ( Fig. 2C ). A long spine, up to 10 µm, arises from the mid of the scale. A second type of scales covers the trunk surface from U55–U75. The shape resembles a balloon, lateral notches are not observed, and the spiny processes are hardly visible. Also, a long spine, up to 10 µm, arises from the mid of the scale. The overall size of the trunk scales is fairly uniform throughout the trunk, but they tend to gradually increase in size from the smaller anterior scales, 5 µm width and 7 µm length at U20, to the larger posterior ones, 7 µm width and 10 µm length at U60. The scales are arranged in an imbricate and staggered manner, and overlap so that the broad and rounded anterior half always is covered by scales in the more anterior row. The scale cuticle is thickest along the anterior margin, which makes this, otherwise covered part of the scale, appear most distinct when visualised with LM ( Fig. 3B–E ). However, SEM clearly shows the true arrangement of the scales, and that only their posterior parts are exposed ( Figs 4G , 5A–I ).

Scales on the terminal furcal branches resemble the trunk scales, but are gradually narrowing towards the distal ends of the branches, and change from shield-shape towards cone-shape and finally take the shape of narrow rectangular plates from U90–U100 ( Figs 2A–C , 5J–K ). The medial spine which is flanked by a pair of short spiny processes at the posterior margin is still present despite the changed and strongly narrowed overall shape of the scales. The furcal branches terminate in adhesive tubes that are covered entirely with scales until their distal tips ( Fig. 5L ).

Internal morphology

The fixation and ethanol storage had unfortunately caused bleaching in the specimens, and internal structures, inclusive pharynx or reproductive organs, could hardly be observed. A weak internal shading indicates that the pharyngeo-intestinal transition is located at the narrowest part of the neck, i.e., around U18. The only distinct internal structures were the paired adhesive glands, which are positioned from U62 and extend into each terminal furcal branch ( Fig. 3I ).

Ecology

The species was collected from muddy sediment in Gerlache Strait, and the Inner- and Middle Basins of Andvord Bay, at depths ranging from 551 m to 701 m. All specimens but one, appeared in the upper 0 to 1 mm sediment. A single specimen (paratype NHMD-1801028) was found in the 3 to 5 mm sediment profile .

Figures

FIGURE 1. Map showing the sampling stations. A. Overview of Antarctica, with the Antarctic Peninsula framed. B. Antarctic Peninsula with sampling area framed. C. Sampling area with stations. Red star indicates the type locality; yellow dots indicate additional stations with Musellifer leasiae sp. nov.

FIGURE 2. Line art illustration of Musellifer leasiae sp. nov. A. Dorsal view. B. Ventral view. C. Close-ups of head scales, anterior-, and posterior trunk scales, and terminal furca scales.

FIGURE 3. Light micrographs showing overviews and details of holotype NHMD-1801023 (A-H) and paratype NHMD- 1801024 (I) of Musellifer leasiae sp. nov. A. Ventral overview. B. Body, anterior, U0-32, dorsal view. C. Body, anterior, U0-32, ventral view. D. Body, median, U26-55, dorsal view. E. Body, median, U28-60, dorsal view. F. Body, posterior, U54-84, dorsal view. G. Body, posterior, U54-86, ventral view. H. Caudal furca branches, U67-100, ventral view. I. Body, posterior, U54-82, focused on adhesive glandular tissue. Abbreviations: agt, adhesive glandular tissue; lcf, locomotory ciliary field; vcb, ventral ciliary bands.

FIGURE 4. Scanning electron micrographs showing overviews and anterior details of paratypes NHMD-1801035 (A), NHMD- 1801030 (B) and NHMD-1801029 (C-G) of Musellifer leasiae sp. nov. A. Left lateroventral overview. B. Head (U0-U15), ventral view. C. Head, U0-U15, right lateral view; framed area marks position of close-up in (F). D. Head, U0-U15, dorsal view; framed area marks position of close-up in (G). E. Close-up of mouth ring and mouth opening. F. Close-up of scales near U9, lateroventral view. G. Close-up of scales near U15, laterodorsal view. Abbreviations: lcf, locomotory ciliary field; mz, muzzle.

FIGURE 5. Scanning electron micrographs showing medial and posterior details of paratypes NHMD-1801035 (A-C), NHMD- 1801029 (D, G, I) and NHMD-1801030 (E-F, H, J-L) of Musellifer leasiae sp. nov. A. U25-40, ventral view; framed areas mark positions of close-up in (B) and (C). B. Close-up of scales near U30, ventrolateral view. C. Close-up of scales near U35, ventrolateral view. D. Close-up of scales near U50, dorsal view. E. Close-up of scales near U50, left lateral view. F. Close-up of scales near U50, ventral view. G. Close-up of scales near U70 and base of terminal furca, dorsal view. H. Close-up of scales near U70 and base of terminal furca, left lateral view. I. Close-up of scales near U70 and base of terminal furca, ventral view. J. Close-up of U80, proximally on terminal furca. J. Close-up of U90, distally on terminal furca. L. Close-up of U100, tip of terminal furca.